Puppy with digestion problems - Seen three vets, nothing has helped... Advice please!

I have a 7 month old Cockapoo who is happy, hyper and normal in every way...except since the week I brought him home he has had terrible bouts of diarrhea, sometimes lasting weeks at a time. Sometimes its just pudding'ish, other times it is soup, and once or twice blood as well (took him to the vet immediately those times) I got him from a breeder, who has tried to be helpful and has even offered to pay for vet bills (so not from a puppy mill)

He has been tested and treated for every parasite in the book. The only one he came back positive for was Giardia, which he has been on panacure for. Even so, this is his 5th round of panacure since I got him...and nothing seems to be helping. (His most recent stool sample came back negative for Giardia) he is also on Tylosin twice a day... also not helping as far as I can tell...

I have him on a good food (Eaglepack Puppy), but when he gets sick, the vet normally recommends putting him on rice (or oats), chicken broth and Medi-cal Gastro... none of which helps.